A radio signal can carry information using both its amplitude and phase, and in modern communication links, multiple instances of both amplitude and phase can also be used in order to transmit multiple bits per symbol using, for example, so called QAM, Quadrature Amplitude Modulation.
Today, QAM generators are found in a great variety of systems ranging from hand-held cellular phones and WLAN equipment to long distance radio trunk stations. All transmitters are based on so called I-Q modulators, in which two analogue base band signals are mixed with a radio frequency, RF, carrier in order to modulate the base band data onto the carrier. The I and Q data are encoded onto 90° phase shifted copies of the RF carrier, and subsequently summed to create a carrier that is modulated in both amplitude and phase.
In radio links, the ratio between the information bandwidth and the carrier frequency, the so called BCR, Bandwidth to Carrier Ratio, is generally quite low, due to bandwidth regulations or other practical bandwidth utilization issues. Recently, there is also an emerging need for modulation technology for use in optical links or systems, in which where virtually no restrictions exist on bandwidth utilization.
However, due to both cost and performance reasons, it can be beneficial to first encode information which it is desired to transmit by optical means onto an electrical carrier or signal via modulation, e.g. QAM, and to then convert the modulated electrical signal into an optical signal. This would provide the advantage of a simplified optical transmitter and receiver structure as compared to a purely optical amplitude and phase modulator.